An exemplary embodiment of this invention is directed to an apparatus and method for testing of Roller Compacted Concrete (RCC), specifically in-situ, real-time monitoring of the RCC's moisture, densities and strength.
The RCC mix includes a damp fill, a mixture of low water content, cement, fly ash and aggregate. The RCC mix is transported to the construction site by trucks or by conveyers quickly after it is mixed. The RCC mix is then dumped and spread in large quantities in 14-inch thick loose lifts using earth moving equipment. Immediately following the placement in the loose lifts, the loose lifts are compacted with a vibratory roller to the densities required by the specifications.
Referring to FIG. 1, when a loose RCC lift 20 is placed over a firm base 22 of a previously compacted RCC lift, and compacted by a vibratory roller 24, it will compress, as shown in FIG. 1. This compression is achieved under confined lift conditions, in which a change of lift volume is due to vertical movement only, i.e. vertical adjustment of granular aggregate particles as a result of combined effort of the vibratory roller's static pressure and dynamic impact.
When a compaction sampler is embedded into a loose RCC lift 20 and compacted it will sink together with the loose RCC lift 20 at the rate equal to the vertical adjustment of lift's granular particles, under the same applied compaction loads.
The lateral shifting of the RCC lift 20, under the applied compaction load and the lift confined conditions is minimal. Compression of the loose RCC lift 20 will occur between a firm base 22 of the RCC lift 20 and the surface 26 of the loose RCC lift 20 at which the compaction load is applied. Compression will be the greatest at the surface of the loose lift at t(s), and it will be reduced with an increase in a depth of the lift 20 below its initial loose surface 26. The compression value will be a smaller at the middle of compacting RCC lift at t(m), and it will reach a minimum value t(b) at the bottom of the lift 20, i.e. at the top of the firm base 22. The compressibility of the loose RCC lift 20, under the same applied compaction load may still vary between the different measuring points. This variation will depend on the variation of the initial density and moisture content of loose RCC lift material at two different locations. However, compressibility along any vertical plane within the loose RCC lift 20 at, and in an immediate vicinity of each measuring point will be the same.
The most important step in the monitoring process of a rapidly placed RCC lift, at various locations is the direct testing of the RCC lift moisture, density and strength, during the actual placing time and under the identical field compaction conditions. At the present time, only the indirect testing methods are being used to determine these compaction parameters, always under the indirect testing conditions that are being approximated to be as the in-situ actual testing conditions of the compacting RCC lifts. The current indirect testing methods provide delayed and inconsistent test results as discussed in the next section. The current indirect testing methods are used for monitoring of the in-situ moisture, densities and strength for the compacted RCC lifts. The indirect testing method for moisture and density measurements may include a nuclear gauge device and laboratory testing of RCC cylinders that are prepared individually during construction, in order to obtain various strengths of compacted RCC material. In order to obtain reliable results with a nuclear gauge, the nuclear gauge must be correctly calibrated to account for variation in composition of the mineral aggregate and its maximum size. Highly siliceous or calcareous aggregate usually produce erroneous readings, if the gauge is not properly calibrated to take into account these variations.
On the other hand, for the material that contains carbons, as bottom ash, calibration of the gauge may not be possible. Nuclear gauge measures the hydrogen in the form of water present in the compacted material. When the compacted material, such as bottom ash, contains naturally occurring hydrogen or bound hydrogen the nuclear gauge will indicate the moisture content falsely high in many cases. Some of the compacted materials showing the higher readings are: fly ash, bottom ash, cement, lime and gypsum. In addition, driving probes into compacted RCC lift that contains larger size aggregate causes some shifting-loosening of aggregate within the compacted RCC lift. This results in reduced density readings particularly in the lower section of the RCC lifts.
Regarding the laboratory testing of the RCC cylinders, the RCC cylinders are prepared in the field at the time of RCC placement. Then, the RCC mix to be compacted in the field under the actual placement conditions during construction is placed in the metal test cylinder and compacted with a metal plunger having a slightly smaller diameter than the test cylinder. The plunger is acted on by an operator or a frame mounted hammer. The test cylinders are rigid with an unyielding side wall. Thus, the compaction of the RCC material in the rigid cylinder with unyielding side wall is dependent entirely on the static compaction effort of the plunger. The vertical adjustment of the aggregate particles is different than the vertical adjustment of the RCC lift that occurs during the actual construction, since the combined compaction effort of the vibratory roller drum's static pressures and dynamic impact for the compaction of RCC material in the test cylinder is more than the compaction effort for the continuous loose RCC lift in the field due to the unyielding side wall.
Further, due to rapid placement of the RCC lifts, testing of the cylinders does not provide concurrent correlation of lift's water contents with lift's densities, i.e. the RCC strength generated within the RCC lift during actual field compaction, with the strength of the RCC mix obtained from the test cylinders which were prepared at the time of placement, but commonly tested after a subsequent RCC lift is placed. In addition, field preparation of RCC cylinders utilizes the compaction effort of either hand held or frame-mounted hammer. Neither of these compaction options generates a specified and consistent amount of compaction energy that is equivalent to the compaction effort generated by full scale compaction equipment during actual compaction of the loose RCC lift in the field.
In the case of the compaction option of using the hand held hammer, the compaction energy is operator dependent.
Therefore, a compaction energy applied to the test cylinders is inconsistent with the compaction energy applied to the RCC lift, in the field, by full scale equipment.
A sand cone method has been used also to obtain density and moisture of a low strength small aggregate RCC material, such as the bottom ash cements mixture.
However, in this method, the RCC mix that contains larger size aggregates is hard to dig after compaction, and there is always a possibility that the volume of the removed material from the hole is not totally accounted for leading to some error in the measurements. In order to minimize this error, correction for the large aggregate must be made.
Hence, it is difficult to equate the densities obtained either by the nuclear gauge, the test cylinders and sand cone with the densities achieved in the RCC lift during an actual field compaction with the full scale compaction equipment.
Methods and Apparatus for testing RCC may benefit from improvements in view of these difficulties.